I know, you’re thinking “why don’t they do it automatically?” – the reason is that the grains will only travel a very short distance into the aerogel, and make marks quite similar to natural cracks that will have appeared during the mission. It is, in fact possible – BUT:

In order for it to work, however, they would have to “train” the computer with real images of aerogel containing grains of interstellar dust. But here’s the rub: no such particles had ever been collected!

So it’s a classic chicken/egg situation – they could find them automatically but they need to find the grains to teach the program before it will work reliably.

So, now it’s time for the public! They will produce a lot (order of a million) images for people to go through one by one (after some online training) to try and spot these particles by eye. The article on the challenge at the Stardust@Home site likens it to:

…searching for 45 ants in an entire football field, one 5cm by 5cm (2 inch by 2 inch) square at a time!